In conventional radical polymerization processes, the polymerization terminates when reactive intermediates are destroyed or rendered inactive; radical generation is essentially irreversible. It is difficult to control the molecular weight and the polydispersity (molecular weight distribution) of polymers produced by conventional radical polymerization, and difficult to achieve a highly uniform and well-defined product. It is also often difficult to control radical polymerization processes with the degree of certainty necessary in specialized applications, such as in the preparation of end functional polymers, block copolymers, star (co)polymers, and other novel topologies.
In a controlled radical polymerization process radicals are generated reversibly, and irreversible chain transfer and chain termination are absent. There are four major controlled radical polymerization methodologies: atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT), nitroxide-mediated polymerization (NMP) and iniferters, each method having advantages and disadvantages.
Atom transfer radical polymerization (ATRP) has been described as a simple, versatile and efficient controlled radical polymerization process. See M. Freemantle, “In Control of a Living Process”, Chemical and Engineering News, Sep. 9, 2002, pp. 36-40. ATRP processes typically employ an alkyl halide as an initiator and a transition metal complex as a catalyst to produce a polymeric radical in the presence of a monomer.
Atom transfer radical polymerization systems based on the combination of a transition metal halide and an alkyl halide have been described. “Atom transfer” refers to the transfer of the halogen atom between the transition metal and the polymer chain. For example, K. Matyjaszewski, (Macromolecules, vol. 28, 1995, pp. 7901-7910 and WO 96/30421) describes the use of CuX (where X=Cl, Br) in conjunction with bipyridine and an alkyl halide to give polymers of narrow molecular weight distribution and controlled molecular weight. A comprehensive review of ATRP is provided by Matyjaszewski and Xia, Chem. Rev., vol. 101, pp. 2921-2990, 2001.
Thus, there is a need for a radical polymerization process which provides (co)polymers having a predictable molecular weight and a narrow molecular weight distribution (low “polydispersity”). A further need is strongly felt for a radical polymerization process which is sufficiently flexible to provide a wide variety of products, but which can be controlled to the degree necessary to provide highly uniform products with a controlled structure (i.e., controllable topology, composition, stereoregularity, etc.). There is further need for a controlled radical polymerization process which provides telechelic (co)polymers capable of entering into further polymerization or functionalization through reactive end-groups, particularly electrophilic end groups.